1. Field of the Invention
The present invention relates to the extrusion of thermoplastic tubular films, in particular the employment of a coextrusion process to produce a tubular film laminate which may be employed in, for example, packaging applications. These applications may employ the present films in laminar form or, alternatively, may be employed in a delaminated form as monolayer materials.
2. Brief Description of the Prior Art
In the past, thin thermoplastic films which are intended for use in packaging, e.g., the fabrication of bags or overwrap packaging materials, have been produced by the extrusion of polymeric resins such as polyolefins in the form of a continuous seamless tubing or as continuous flat sheeting. In the latter instance, molten polymer resin is extruded through a slot die orifice onto the surface of a rotating drum or chill-roller. In the case of tubular film extrusion, employing a standard rotating screw extrusion apparatus, molten polyethylene is formed within the extruder and is extruded under pressure through an annular orifice into a tubular extrusion die. Air, under pressure, is introduced through the die and into the extruded tubing and causes the tubing to inflate. The air under pressure is entrapped within the inflated tubing, forming a bubble, by a pair of counter rotating, positively driven nip rollers located downstream of the tubular extrusion die. These rollers flatten the film tubing and tightly nip together opposing surfaces of the tube thereby maintaining the air, under pressure, encapsulated within the tubing throughout the extrusion operation. As the molten polymer leaves the tubular die, air rings which surround the tube adjacent to the die may be employed to cool the advancing molten polymer as it is being inflated and cause it to solidify prior to being nipped together downstream by the nip rollers. The resultant flattened tubing as it emerges from the nip rollers is subsequently fed to fabricating equipment, such as bag-making machinery and the like.
During the tubular extrusion, cooling forces acting upon the molten tube as it emerges from the die orifice result in solidification of the molten polymer prior to its being collapsed into a flattened tube by the positively driven nip rollers. Conventional air rings which surround the advancing tubing as well as ambient air temperature all take part in this cooling operation. Those skilled in the art are aware that, in the tubular extrusion process, there is a visibly defined transition point in the extruded tube somewhere between the die and nip rollers. This visible line which circumscribes the tube is referred to as the frost-line and is observed at that point along the length of the tubing where the transition from a semi-molten to a solid film occurs. This frost-line will appear around the tube at that point where the film bubble reaches the point of maximum diameter. In the case of the coextrusion of thermoplastic materials having a significant melting point differential, and where the higher melting point material forms the external layer of the tubing, the external molten layer will be the first to freeze, i.e. turn from a molten state to a solid state. This frozen external layer will then become a solid support for the lower melting point inner layer which remains in a molten or semi-molten state until it advances further downstream of the higher melting point material frost-line where it eventually solidifies, prior to entry into the nip rollers.
Drawing or stretching forces are continuously acting upon the polymer tubing intermediate the annular die orifice and the nip rollers. Some of these forces include a draw-down of the molten or semi-molten polymer by virtue of its being drawn from the die orifice by the positively driven nip rollers. The draw-down force acting on the film tube is significant as illustrated by the fact that the thickness of the molten material which emerges from the die orifice may be as high as 50 mils and the thickness of the final film layer may be on the order of 1 mil, which means that the draw-down ratio may be on the order of 50 to 1 or higher, in normal extrusion operations. Obviously, such ratios will vary depending upon factors such as the material being extruded, the extrusion equipment employed, the desired gauge of the final film produced, and the like. In the case of the extrusion of materials such as high density polyethylene, this draw-down force will impart a high degree of orientation to the polymer in the direction in which the draw-down occurs, i.e., the longitudinal or machine direction or direction in which the tube is advancing. Polymeric materials, such as high density polyethylene for example, are comprised of very long chain molecules, and the drawing or stretching of such materials when they are at elevated or orientation temperatures induces an alignment of these relatively long molecular chains in the direction of the draw. In the case of tubularly extruded high density polyethylene, such orientation produces a film which has a very high tensile modulus (stiffness) and strength in the machine direction or direction of draw. This is highly desirable where such material is to be employed in packaging and bag-making applications. However, such machine direction molecular alignment or orientation results in such films having a tendency to split or rupture when tensile forces are applied transversely to that machine direction. This apparently insurmountable problem attendant to and inherent in conventional prior art tubular extrusion techniques for these types of materials has resulted in a narrowing of the scope of end use applications available for such materials.
In addition to the draw-down force which acts upon the tubing being extruded hereinbefore described, other drawing forces including forces which extend the tube in a direction transverse to the machine direction while simultaneously imparting additional machine direction draw, are at work during the tubular extrusion process. These forces are a result of the tubing being inflated by the entrapped air bubble, such inflation pulling the tube both in the machine or longitudinal direction as well as expanding it transversely. The amount of tubular inflation is precisely controlled during tubular extrusion, dependent on the end use property requirements of the tubing which is being fabricated. The degree of inflation is referred to as the BUR or the blow-up ratio.
The transverse direction stretching by tube inflation orients the crystalline polymer in the transverse direction although not to the degree which the tube is machine direction oriented, i.e., the orientation is not balanced and the tube, as a result, remains splitty in the machine direction.
In the case of certain monolayer films which are tubularly extruded it has been found that reducing the blow-up ratio below about 4 to 1 usually results in films which have low resistance to transversely applied tensile forces, i.e., they are quite splitty or susceptible to tearing in the machine direction. This lowering of the BUR, lowering the amount of transverse direction stretching, increases the imbalance of orientation draw ratios and, accordingly, aggravates the machine direction splitting problem. Additionally, it has been found that increasing the rate of extrusion, i.e. speed of tube production, also results in a lowering of machine direction tear resistance. It has now been found, in accordance with the present invention, that when lower melting point polymers are extruded in the form of a tube and that tube is externally supported by a coextrusion of another higher melting point resin, the undesirable effects of BUR reduction and increased operating speeds normally encountered when these polymers are extruded is either eliminated or substantially reduced. While the exact mechanism which occurs to cause this phenomenon is not fully understood, it is theorized that since the higher melting polymer will freeze or solidify at a higher temperature than the lower melting point polymer, the higher melting polymer forms a solid, non-molten support structure which encases the inner polymer tube and either stops or retards the draw-down of this material at that point. If this in fact occurs, the inner, lower melting point, material remains molten for a period of time after its initial draw-down is completed, supported by the external solid tube, thereby allowing time for the molecular orientation imparted by the draw-down to relax before the crystalline tube solidifies. Such relaxation will reduce the degree of orientation in the final crystalline film product. Accordingly, since there is less orientation, the deleterious affects of unbalanced orientation on film properties is reduced. And since the unbalanced orientation is responsible for machine direction splittiness, such splittiness is thus reduced.